I've used BB since it was invented and after a couple of side-by-side experiments many years ago give the plants a half dose a week before flipping to flower then the other half at the flip along with more of the Bloom but don't switch to full Bloom feed until after the stretch as the stretch is a huge growth spurt where lots of all nutes are needed. Once stretch is over I switch to a Lucas Formula type feed where I cut out the Grow and use Micro at half the amount I use Bloom to hit my ppm target as close as I can. For a 9 - 10 week flowering strain I stop using the BB at the end of week 6.
I prefer the powder form of BB as it has a much longer shelf life. I'd keep the liquid one in the fridge if you don't use it all up in one grow.